1. Field of the Invention
The invention relates to systems for electrical transmission between two units that are movable relative to each other. These are used, for example, in the form of linear slide tracks in linearly movable units such as crane systems, or as sliprings for rotating data transmission, for example in machine tools or computer tomographs.
2. Description of the Related Art
Both contacting and non-contacting transmission techniques are known for sliprings or slide tracks. Because sliprings for rotating data transmission, and linear or curved slide tracks operate on the same technical principle, no distinction will be made between them in the following exposition. The terms will be used as being synonymous.
The manner of operation of electrical signal transmission will be briefly explained in the following, using a slipring as an example. Circular or annular slide tracks are mounted in a first unit. A second unit moving relative to these has a sliding contact resting upon a slide track with which it is in sliding contact during the movement. An electric current can now be transmitted via this galvanic contact. As an alternative to a sliding contact, a non-contacting tap is also possible. In this case, the coupling is preferably capacitive or inductive, for example via a field probe. Non-contacting transmissions of this kind are preferably used for medium to high frequencies. Although the term “slide tracks” is being used here, it is intended to apply to those which are basically suitable for non-contacting or for contacting transmission. Furthermore, no distinction will be made between transmissions of electrical signals and energy, because in both cases the mechanisms of transmission are basically the same.
Particularly in the case of medium and high frequency signals, a problem with the arrangements is that of transmitting these signals along a slide track with as little interference as possible. With slide tracks as normally used, only electrical signals can be transmitted that have a wavelength which is large compared with the electrical path along the slide tracks. With small devices, for example with sliprings having diameters of only a few millimeters, as a rule the length of a slide track can be neglected, so that high bandwidths may be achieved here. With large arrangements, for example with sliprings as used. in computer tomographs and having a circumference of more than 5 meters, only relatively low bandwidths can be achieved.
A solution described in the U.S. Pat. No. 5,018,174 improves the transmission characteristics of the slide tracks. Here a terminal resistor is disposed at a position diametrically opposite to the feed-in position. The disadvantage of this arrangement is that in fact no reflection-free termination can be achieved because of this so-called terminal resistor. Reflections still occur at the position of the so-called terminal resistor and lead to interference with the signal. Furthermore, and particularly with feeding-in of signals from the second unit, a further movable tap (brush or brush block) must be provided on the second unit. This further tap is disposed to be diametrically opposite (displaced by 180 degrees from the coupling-in position). Thus, with an arrangement of this kind two movable taps are always needed. This results in increased costs in production, and increased wear in the case of a sliding contact arrangement. In fact, any advantage provided by the increased outlay is only very limited.
Furthermore, non-contacting transmission systems are known from prior art, for example from DE 44 12 958, which have no closed conductor path. Thus, with these the conductor path is interrupted at least at one position. This is acceptable for non-contacting transmission. These conductor paths may be terminated to be free from reflections by connecting to each end a resistor corresponding to the characteristic impedance of the conductor path. With this, a signal fed-in at an arbitrary location along the conductor path can propagate along both directions, starting from the feeding-in location, as far as each one of the terminated ends. There it will be finally absorbed and can therefore no longer be reflected back into the conductor path. If the signal were to be reflected, then it would be received a second time by the receiver. Multiple receptions of this kind can result in an impairment extending up to massive interference with the transmission. However, slip rings which preferably have a brush running along a slide track cannot be manufactured economically to include a discontinuity Thus, for example, increased brush wear will occur at the site of the discontinuity. Furthermore, a closed track is preferred also for reasons of fabrication technology, because with this, mechanical strains are uniformly distributed around the circumference. Finally, when a brush passes a site of discontinuity of this kind, a short-circuiting of the discontinuity will occur and at least for a brief moment of time lead to exactly the signal interference which it was intended to avoid. In the case of high data rates of several 100 Mbit/s, this can lead to large losses of data.